If $5a + 2b + 7c = 4$ and $x + 2y + 9z = -4$, what is $9x - 7c + 81z - 5a - 2b + 18y$ ?
Answer: $= -5a - 2b - 7c + 9x + 18y + 81z$ $= (-1) \cdot (5a + 2b + 7c) + (9) \cdot (x + 2y + 9z)$ $= (-1) \cdot (4) + (9) \cdot (-4)$ $= -4 - 36$ $= -40$